Optical pulse transmitter

ABSTRACT

It is disclosed a method for driving a laser diode such as to enable mitigation or elimination of so called spiking effects related to the number of injected carriers in the laser overshooting the equilibrium value at the beginning of the lasing process. In this manner, among other things, the efficiency of a master oscillator power amplifier that may be utilized in range finding applications will be improved. It is further disclosed an optical pulse transmitter comprising such a laser diode.

TECHNICAL FIELD

The present invention generally relates to range finding. In particular,the present invention relates to a method of driving a laser diode andan optical pulse transmitter.

BACKGROUND ART

The art of surveying, or range finding, involves the determination ofunknown positions, surfaces or volumes of objects using measurements ofangles and distances. The determined angles and distances from ameasuring instrument to points under survey may be used to calculate thecoordinates of surveyed points relatively the measuring instrument. Inorder to make these measurements, an optical surveying instrument orgeodetic instrument frequently comprises an electronic distancemeasuring (EDM) device which may be integrated in a so-called totalstation, see FIG. 1. A distance measuring total station combineselectronic and optical components and is furthermore in general providedwith a computer or control unit with writable information forcontrolling the measurements to be performed and for storing dataobtained during the measurements. Preferably, the total stationcalculates the position of a target in a fixed ground-based coordinatesystem. Such a total station may comprise a telescope, which for examplemay be arranged with cross-hairs for sighting a target. Angle ofrotation of the telescope, as well as angle of inclination of thetelescope, may be measured relative to the target. Total stationscomprising a camera are also known.

In conventional EDM, a light beam is emitted as a light pulse towards atarget, and light reflected by the target is subsequently detected atthe optical surveying instrument, such as a total station. Processing ofthe detected signal enables determination of distance to the target bymeans of, e.g., time-of-flight (TOF) or phase modulation techniques.Using the TOF technique, the time of flight of a light pulse thattravels from the surveying instrument (the EDM device) to a target, isreflected at the target and returns to the surveying instrument (the EDMdevice) is measured, on the basis of which distance may be calculated.The power loss of the received signal determines the maximum possiblerange. Using a phase modulation technique, light of differentfrequencies is emitted from the surveying instrument to the target,whereby reflected light pulses are detected and the distance iscalculated based on the phase difference between emitted and receivedpulses. As mentioned in the foregoing, once the angles and distanceshave been measured, the actual position of a surveyed target may becalculated.

In a conventional scanner, for example intended for use in industrial,surveying and/or construction applications, or in other applications,the light beam may be guided over a number of positions of interest atthe surface of the target using a beam steering function. A light pulseis emitted towards each of the positions of interest and the light pulsethat is reflected from each one of these positions is detected in orderto determine the distance to each one of these positions. For example,using a LIDAR (Light Detection and Ranging) scanner, properties ofscattered light may be measured to find range and/or other informationof a distant target. In general, the distance to an object or surface isdetermined using laser pulses.

For increasing the measurement range in the TOF ranging applications,use of a master oscillator power amplifier (MOPA) may be advantageoussince a high peak power can be achieved in the transmitted pulse, thusresulting in a longer range and higher measurement rate due to a highersignal-to-noise ratio. Higher output power is also advantageous in thephase modulation systems for the same reason. In a MOPA, a master laseris employed in combination with an optical amplifier used to amplify theoutput of the master laser. The master laser is often referred to as aseed laser. By using an optical amplifier to boost the output, therequirements on the seed laser may be mitigated, which allows reachinghigher wavelength stability and spatial quality of the beam for thetransmitter. A particular type of MOPA is realized with a microstructuresemiconductor seed laser diode and an optically pumped fiber amplifier,which sometimes is referred to as a master oscillator fiber amplifier(MOFA).

To reach a high enough accuracy of distance measurements, short pulsesshould be used. Normally, optical pulses with duration t_(p) of 1 to 50ns are used, depending on application. Transmitters utilizingsubnanosecond pulses are also known (cf., e.g., S. N. Vainshtein et.al., Rev. Sci. Instrum. vol. 71, no. 11, p. 4039-4044 (2000)). Toprovide optical pulses of duration of t_(p), the carrier life time inthe laser t_(L) should be about t_(p) or shorter.

Let us assume that we want to obtain a 1 ns long optical pulse from themicrostructure laser and that a laser with carrier lifetime t_(L)<<1 nsis used. Using a microstructure laser diode as a seed laser requires anappropriate electrical laser driver, in particular, a pulsed laserdriver for TOF applications. Driving the seed laser with nanosecond orsub-nanosecond electrical pulses causes an intensive relaxationoscillation process, also called “spiking” in case relaxationoscillations are limited to one pulse, before lasing can be established.The origin of relaxation oscillations is directly related to therecombination processes in semiconductors (see, e.g., chapter 4 in“Handbook of semiconductor lasers and photonic integrated circuits”, Ed.Y. Suematsu and A. R. Adams, Chapman & Hall, 1994). In particular,characteristic time and amplitude of relaxation oscillations depend onspontaneous and stimulated emission relaxation times t_(s) and t_(ph),as well as on number of carriers and photons (see, e.g., p. 266 in“Handbook of semiconductor lasers and photonic integrated circuits”, Ed.Y. Suematsu and A. R. Adams, Chapman & Hall, 1994).

According to Einstein's quantum theory of light, there are twocategories of light emission processes (also described in the book ofSuematsu and Adams). The transition probability of the first kind oflight emission process is proportional to the existing photon density.This is called the stimulated emission process. The transitionprobability of the second kind of light emission process is independentof the photon density and is called the spontaneous emission process.When applying a short current pulse to the semiconductor laser diode, alarge number of carriers are injected in the active area. If theconcentration of carriers is high enough, which is above the thresholdlevel, population inversion is achieved, wherein stimulated emissioncommences, which in turn results in a growing number of photons, i.e.the lasing starts. However, the density of photons of the first kind,corresponding to the stimulated emission, is close to zero in thebeginning of the process, and growing of the number of photons is veryslow. Because of that, the increasing concentration of carriers underthe current pumping does not immediately result in an increase of thephoton density and the concentration of carriers overshoots the levelcorresponding to the equilibrium lasing condition, under which thegrowth in the number of carriers is compensated by the radiativerecombination process. After the photon density becomes large enough tointensify the optical recombination process, the number of carriersdrops down to the equilibrium level and a spiking pulse is emitted fromthe seed laser. The spiking pulse is amplified by the fiber amplifierand therefore is present in the transmitted signal together with themain, intended optical pulse. The desired pulse shape is therebydistorted which may cause a decrease in the distance measurementaccuracy.

While parameters of the main pulse are determined by the amplitude andduration of the driving pulse, both amplitude and start time of thespiking pulse are less predictable. In addition, the spiking pulse has aspectrum different from that of the main pulse, and the spiking pulsemight further be spatially different from the main pulse, which maycause additional errors in the distance and angle measurements.Therefore, it is desired to eliminate—or at least mitigate—the spikingprocess.

The spiking, or relaxation oscillations, may be mitigated in differentways. First, the laser may be driven in continuous wave (CW) mode, whilethe optical power is varied by means of a modulator, for example anacousto-optic modulator. This approach suffers from limited extinctionratio at the output as well as high insertion loss to the modulator.Also, the maximum output power is limited.

Another, more general, approach is to develop laser diodes producingless relaxation oscillation by optimizing the laser structure. Forinstance, width and length of the optical resonator has large impact onwavelength and power stabilizing. Presently available semiconductorlaser diodes have highly optimized structure, so that they may providesingle-mode output in a wide range of driving current. Thatsignificantly reduces relaxation oscillations while driving the laserabove the threshold, but spiking is still present if the driving currentrises sharply from zero.

A third way to mitigate spiking is to continuously operate the seedlaser above the lasing threshold, though not in the CW mode. For rangingapplications, however, such a solution is typically not an option.First, most ranging instruments and devices are battery-driven, thus ingeneral having a limited supply of energy, while driving the laser abovethe threshold adds significantly to the power consumption. Second,continuous driving of the laser above the threshold causes continuousillumination of the target between the pulses, though at lowerintensity, which, in turn, may reduce contrast and decrease measurementaccuracy. The third and most serious drawback of continuously operatingthe laser above the threshold applies to a seed laser in combinationwith an optically pumped fiber amplifier as discussed in the above—thecontinuous application of a bias current exceeding the lasing thresholdgenerally results in poor efficiency.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above mentionedproblems and to provide a method and apparatus for driving a laser diodesuch that the effect of relaxation oscillation described above ismitigated.

To this end, there is provided a method of driving a laser with apre-formed current pulse consisting of two pulses, a main driving pulseand a preceding pulse having a lower amplitude than that of the maindriving pulse, for creating a light beam emitted as a light pulsetowards a target, the distance to which is to be measured by a surveyinginstrument in which the laser is incorporated. The signal preceding themain drive signal will in the following be referred to as the pre-drivesignal. The generated pre-drive signal may be formed such that spikingeffects are mitigated both at the laser optical output and at the outputof a fiber amplifier.

In a semiconductor laser, an applied current step above the thresholdlevel will cause population inversion, whereby stimulated emission willcommence. However, since the stimulated optical transition probabilityis proportional to the number of photons, which may be very low at thestart of the current pulse, the population inversion level maytemporarily exceed the equilibrium level. After the number of photonsbecomes large enough, the optical recombination process intensifies andthe number of carriers falls down to the equilibrium level, whereby aspiking pulse is emitted. The larger the deviation of the number ofcarriers from the equilibrium level, the more intense the spiking effectwill be. The increase in the stimulated emission is effectively delayedrelatively the current step by the time needed to create a large enoughnumber of stimulated emission photons. This characteristic time, t_(c),is specific for a particular laser and depends on a number of parameterssuch as carrier recombination time, time for establishing an opticalmode in the resonator etc. The time t_(c) is also closely related to theinverse of the critical modulation frequency ω_(c) ⁻¹: The higher themodulation speed that the laser can provide, the shorter thecharacteristic time t_(c).

In order to mitigate or eliminate the spiking effects, the pre-drivesignal should be formed such that its current level is kept above thelaser current threshold value for such duration of time that the numberof carriers injected in the laser approaches an equilibrium level beforethe main drive signal is applied.

The pre-drive signal of the present invention results in a slow increasein the number of injected carriers up to a value close to the laserthreshold value, at which the number of stimulated emission photons isalready large enough for making the delay between the current change andthe change in the number of photons very short. Subsequently, when themain current pulse is applied, the effect of overshooting with respectto number of injected carriers is small and spiking may effectively bemitigated or avoided.

The inventors have realized that by applying the pre-drive signal to thelaser prior to applying the main drive signal of the laser, undesiredrelaxation oscillation pulses present in the optical output may beeliminated.

Further, there is provided an optical pulse transmitter which is drivenwith a pre-drive signal and a main drive signal as discussedhereinabove.

The laser may be operated in either of a single mode of its resonator orin multiple modes of its resonator. Multi-mode operation of the laserenables for example reaching a higher power compared to a single-modelaser. In this manner, an optical amplifier may not be required inoperating the optical pulse transmitter.

According to an exemplifying embodiment, the pulse generator of theoptical pulse transmitter may be adapted to form the pre-drive signalsuch that a resulting optical output signal has an energy content thatis less than a predetermined fraction of the energy content of anoptical output signal resulting from the main drive signal.

Such a configuration enables for example to reduce the energyconsumption of the optical pulse transmitter. At the same time, spikingeffects may be mitigated or eliminated. The predetermined fraction maybe chosen on the basis of choice of application and/or capacityrequirements. For example, in some applications the predeterminedfraction may be about 10%. In other applications the predeterminedfraction may be considerably lower than 10%.

In an exemplifying embodiment, the optical pulse transmitter comprisesan optically pumped amplifier for amplifying a signal provided by thelaser diode, thereby producing an optical output signal of the opticalpulse transmitter. The laser diode may thus be employed in combinationwith an optical amplifier. The combination of the laser and the opticalamplifier may for example constitute a master oscillator power amplifier(MOPA).

In an exemplifying embodiment, the optical pulse transmitter, or theoptically pumped amplifier, comprises a doped fiber amplifier.

The doped fiber amplifier may be adapted to operate in an eye-safewavelength range. For example, the doped fiber amplifier may be anEr-doped fiber amplifier (EDFA) operating in the wavelength range fromabout 1530 nm to 1565 nm where eye-safe operation of the laser ispossible at high peak power. In general, the doped fiber amplifier maybe doped with one or more rare-earth metals, such as Neodymium (Nb),Ytterbium (Yb), Erbium (Er), Thulium (Tb), Praseodymium (Pr) and Holmium(Ho), and may be adapted to operate within the wavelengths of about1.03-1.10 μm, 1.0-1.1 μm, 1.5-1.6 μm, 1.45-1.53 μm, 1.3 μm and 2.1 μm,respectively. The doped fiber amplifier may be doped with anycombination of rare-earth metals.

In the context of some embodiments, by “eye-safe” operation of lasers itis referred to operation of the lasers in a wavelength range where thelight emitted from the laser for example cannot penetrate the cornea ofa human, thus protecting the retina from damage by the laser light.

The duty cycle of the main drive signal may be less than about 1percent. Such a configuration may be particularly advantageous for rangefinding applications.

The duration of the pre-drive signal may be longer than a characteristictime of the laser. As discussed above, the characteristic time of thelaser is generally determined by the time needed to create a largeenough number of stimulated emission photons.

The pulse generator may be adapted to form the pre-drive signal suchthat a resulting optical output signal (i.e. resulting from thepre-drive signal) has an amplitude not higher than about 20% of theamplitude of the output optical signal resulting from the main drivesignal.

Alternatively or optionally, the pulse generator may be adapted to formthe pre-drive signal such that a resulting optical output signal (i.e.resulting from the pre-drive signal) has a duration that is not longerthan about 20% of the period of the output optical signal resulting fromthe main drive signal. Further features of, and advantages with, thepresent invention will become apparent when studying the appended claimsand the following description. Those skilled in the art realize thatdifferent features of the present invention can be combined to createembodiments other than those described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in more detail in the followingwith reference made to the accompanying drawings, on which

FIG. 1 shows an example of a prior art total station in which thepresent invention can be applied;

FIG. 2 illustrates the effect of spiking at the output of an opticalamplifier;

FIG. 3 illustrates an example of a pre-drive signal and a main drivesignal;

FIG. 4 shows numerical simulations of carrier density and output opticalpower for a semiconductor laser;

FIG. 5A shows an example of an optical pulse transmitter in accordancewith an embodiment of the present invention;

FIG. 5B shows an example of an optical pulse transmitter in accordancewith another embodiment of the present invention;

FIG. 6 shows an example of a pre-drive signal, a secondary drive signaland a main drive signal;

FIG. 7 shows an example of an optical pulse transmitter in accordancewith a further embodiment of the present invention; and

FIG. 8 illustrates an example of a pre-drive signal and a main drivesignal generated in the optical pulse transmitter of FIG. 7.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

First of all, a prior art surveying instrument in the form of a totalstation, in which the present invention can be applied, will be brieflydescribed.

In FIG. 1, there is shown an example of a prior art total station 100comprising an alidade 101 mounted on a base 102, and having a mountingsupport structure in the form of a tripod 103. The alidade 101 can berotated about a vertically oriented rotation axis V, in order to aim theinstrument in any desired horizontal direction. In the alidade, there isarranged a center unit 104, which can be rotated about a horizontallyoriented rotation axis H, in order to aim the instrument in any desiredvertical direction. Measurements made using the total station 100 aretypically related to an origin of coordinates located at theintersection between the vertically oriented and the horizontallyoriented rotation axes V and H.

For rotation of the alidade about the vertically oriented rotation axisto aim the instrument in any desired horizontal direction, there isprovided drive means 105. The rotational position of the alidade 101 istracked by means of a graduated disc 106 and a corresponding angleencoder or sensor 107. For rotation of the center unit 104 about thehorizontally oriented rotation axis, there are provided similar drivemeans 108, graduated disc 109 and sensor 110. Moreover, the instrumenthas an optical plummet 112, which gives a downwards view along thevertically oriented rotation axis. The optical plummet is used by theoperator to center or position the instrument above any desired point onthe ground. The instrument can also be manually operated for aimingtowards a desired target using vertical and horizontal motion servoknobs 115 and 116. The means for rotating the instrument is preferablyimplemented in association with the servo control system for theinstrument for controlled activation of the drive motors 105 and 108.

The instrument line of sight is centered at the intersection between thevertical and the horizontal rotation axes, and this can be seen in thefigure where these axes cross in the center of a telescope 113 in thecenter unit 104.

In the telescope, there is provided a light source 114, such as a laserpointer. Preferably, the light source is coaxial with the telescope,even though it could be placed elsewhere such that it instead iseccentric with the telescope. It should be understood that light sourceshaving other wavelengths, e.g. light being invisible to the human eye,may be used. The light source is used for indicating an object to betargeted, in order to perform EDM. The instrument may also comprise adisplay device for showing the indicated target. The display may be anintegral part of the instrument, but more preferably, the display isincluded in a removable control panel that can be used for remotecontrol of the instrument via short range radio. It is even conceivablethat the instrument is fully remote controlled, wherein the display maybe in the form of a computer screen located far away from the totalstation, and wherein information to and from the instrument aretransferred over a wireless computer or radio telephone network.

The instrument can also be manually operated for aiming towards adesired target using vertical and horizontal motion servo knobs 115 and116.

The means for rotating the instrument is preferably implemented inassociation with the servo control system for the instrument forcontrolled activation of the drive motors 105 and 108.

FIG. 2 illustrates the effect of relaxation oscillations at the outputof an optical amplifier. As has been previously described, driving aseed laser with nanosecond or sub-nanosecond electrical pulses may causea relaxation oscillation process before lasing is established. Whenapplying a current pulse to the laser diode, a large number of carriersare injected in order to achieve population inversion, whereinstimulated emission commences and lasing starts. The relaxation processcauses one or more ultra-short spiking pulses on the leading edge of themain pulse which is output from the seed laser and, after amplificationin the fiber amplifier, the MOPA itself, see FIG. 2. As can be seen fromFIG. 2, the duration of the spiking pulse is fairly short and its edgesare rather steep as compared to the main optical pulse. While parametersof the main pulse are determined by the seed laser driver, bothamplitude and start time of the spiking pulse are rather defined byinternal laser parameters like carrier recombination time,multiplication rate of photons, etc. In addition, the spiking pulse mayhave a spectrum different from that of the main pulse, and it mayfurther be spatially different from the main pulse. This may causeadditional errors in distance measurements. Therefore, it is desired toeliminate—or at least mitigate—the spiking.

In telecommunication applications, this problem is addressed bycontinuously driving the seed laser above the threshold. As aconsequence, carriers are injected to such an extent that the resultingcarrier density does not fall below the lasing threshold.

For ranging applications, however, such a simple and straightforwardsolution is typically not an option. Firstly, many ranging applicationsuse battery-driven devices where energy-saving is essential. Secondly,continuously illuminating a target, though at low intensity, may reducecontrast and, consequently, measurement accuracy. The third and mostserious drawback of continuously driving the laser above the thresholdapplies to using a seed laser in combination with an EDFA as discussedin the foregoing. In case of using a MOPA comprising a seed lasercombined with an EDFA, the continuous application of a bias currentexceeding the lasing threshold generally results in a very poorefficiency, as will be shown in the following.

Consider an example of a MOPA with rare-earth metal doped fiberamplifier (XDFA) and a seed laser generating rectangular optical pulsesof duration t=1 ns, period T=10 μs, and amplitude varying between P₀ andP₁. For an ideal XDFA with gain G, average output power P_(av) isP _(av) =G*(P _(0*)(T−t)+P ₁*t)/T.  (1)In case of P₀=0, the optical peak output power isP _(pk) =G*P ₁ =P _(av)*(T−t)/t≈10⁴ *P _(av).  (2)For a non-ideal XDFA, the amplitude of an optical pulse at the outputwill be slightly reduced due to spontaneous emission (i.e. noisegeneration) in the amplifier. Still, high signal-to-noise ratio can bereached with a correctly designed XDFA.

A seed laser which is permanently driven above its threshold willaccordingly produce a permanently present optical signal at its outputP₀>0. Since this optical signal forms an input signal for the XDFA, theoutput peak power of the XDFA will be dramatically reduced. This is dueto the fact that the average output power of the XDFA is determined bythe pump power which is independent of the optical input signal. Forexample, assuming that P₀ is 10% of P₁ at the output of the seed laser(i.e. the extinction ratio is 10), the peak optical power at XDFA outputcan be calculated asP _(pk) =P _(av) *T/(t+0.1*(T−t))≈P _(av) *T/(0.1*T)=10*P _(av).  (3)Thus, the output peak power will be reduced by a factor 1000 compared tothe result achieved in (2) obtained for P₀=0. Further, nearly all, about99.9 percent, of the optical energy emitted by XDFA will be in theparasitic CW signal, which results in an unacceptably low efficiency foruse in a battery-driven application.

To increase the efficiency of the MOPA, the seed laser has to be drivenwith short current pulses starting from a value below the threshold, forexample zero, that is in return-to-zero mode. However, this causes aspiking problem. To overcome this problem, the seed laser can be drivenaccording to the method of the present invention, where a pre-drivesignal is applied to the seed laser before the main drive signal isapplied.

An example of a main drive (current) signal with a pre-drive signal isgiven with reference to FIG. 3. The duration of the pre-drive signal isapproximately 3.5 ns and its amplitude is 0.2 a.u., whereas the maindrive-signal has a duration of about 1 ns and an amplitude of 1 a.u. Asdepicted in FIG. 3, the pre-drive signal may have an amplitude slightlyabove the threshold current value for the seed laser. The characteristictime t_(c) of the seed laser should be much less than 1 ns, otherwisethe present driving signal will result in a distorted, much sloweroptical pulse.

In the particular example of FIG. 3 and assuming t_(c)<<1 ns, the energycontent of an optical pulse resulting from the pre-drive signal is about15% of the total energy content of the main and pre-drive signals, whichmeans that about 15% of the optical energy output of the XDFA is wasted.

Obviously, the shorter the pre-drive pulse is, and the lower amplitudeit has, the higher the efficiency of XDFA. The lowest amplitude of thepre-drive current pulse is defined by the threshold value. In the bestcase, the pre-drive current should increase to the threshold value orjust above it in order to create a sufficiently large number of photonsto intensify the stimulated emission process. The lower limit for theduration of the pre-drive signal is determined by the characteristictime t_(c) of the particular laser. If the duration time of thepre-drive pulse becomes shorter or comparable to t_(c), the effect ofthe pre-drive signal diminishes and then disappears. The upper limit forthe pre-drive pulse duration is determined by the desired efficiency ofthe whole system.

According to an exemplifying embodiment of the present invention, theduration of the pre-drive pulse may be about the same as the duration ofthe main drive pulse. In that case, the optical efficiency of XDFA wouldbe maximized even in the case of relatively high amplitude of thepre-drive current due to a very short pre-drive pulse. An even shorterpre-drive pulse is, in principle, possible, but generally requiresunnecessarily fast driving electronics (e.g. associated with relativelyhigh costs).

The effect of the pre-drive signal on spiking amplitude will bediscussed in the following. Assuming that a current step with a maximumvalue I_(pk)>I_(th) (where I_(th) is the threshold level, or thethreshold current value of the seed laser) is applied to the seed laser.The carrier density N_(e) will increase with the applied current untilit reaches the lasing threshold value N_(e) _(—) _(th). At this point,population inversion is achieved, wherein stimulated emission commences,which in turn results in a growing number of photons. However, thedensity of stimulated emission photons is close to zero in the beginningof the process, and growing of the number of photons is very slow.Because of that, the increasing concentration of carriers under thecurrent pumping does not immediately result in an increase of the photondensity, and the concentration of carriers overshoots the levelcorresponding to the equilibrium with the number of emitted photons.After the photon density becomes large enough to intensify the opticalrecombination process, the number of carriers drops down to theequilibrium level and a spiking pulse is emitted from the seed laser.The higher the amplitude of the current step, or the larger thedeviation of the number of injected carriers from the threshold valueN_(e) _(—) _(th), the more intense spiking will occur.

To estimate the amplitude of the spiking pulse, a number of laserparameters have to be known, such as the confinement factor, the opticalrecombination time, the diffusion coefficient, etc. To illustrate theeffect of spiking, numerical simulations of carrier density and outputoptical power for a semiconductor laser have been made using a modelincluding rate equations for the number of carriers and photons. Theresult for a Gaussian driving pulse without pre-drive is presented inFIG. 4.

With reference to FIG. 4, on the left-hand side of the drawing, denoteda), the carrier density and the optical power of the laser is presentedfor an applied current pulse (“pumping current”) starting from zeroamplitude. The number of injected carriers increases drastically at 6ns, when the current pulse is applied, and the caused spiking is clearlyobserved in the optical power curve. On the right-hand side of thedrawing, denoted b), the current pulse is starting from a value of 12just below the threshold value (which is subtle in FIG. 4 due to scalingeffects), which results in a slow increase of the number of injectedcarriers to a value close to the threshold value. At 6 ns, when the maincurrent pulse is applied, the effect of overshooting with respect tonumber of injected carriers is small and spiking is effectively avoided.In FIG. 4, all quantities on the vertical axes are given in arbitraryunits.

Thus, to eliminate the spiking effect, the current in the pre-drivesignal should be kept close to the threshold value for such duration oftime that the number of injected carriers practically reaches theequilibrium level. In that case, the arriving main pulse will not causespiking. Preferably, the current in the pre-drive signal should be keptat a value equal to or higher than the laser current threshold value forsuch duration of time that the number of carriers injected in the laserapproaches the equilibrium level as closely as possible, e.g. as closelythat is possible and/or desirable to implement. For example, the currentin the pre-drive signal may be kept at a value equal to or higher thanthe laser current threshold value for such a duration of time that thenumber of carriers injected in the laser approaches the equilibriumlevel within about 10% of the equilibrium level.

FIG. 5A shows an example of an optical pulse transmitter 500 inaccordance with an embodiment of the present invention. A triggeringdevice 501 is employed to initiate drive signals for a seed laser, orlaser diode, 508. By using electrical delay lines 502, 503, pulsegenerators 504, 505 can be controlled with respect to timing ofgenerated pulses, wherein the timing of the pre-drive signal and themain drive signal supplied to the seed laser via current sources 506,507 can be carefully chosen. The triggering device generates a triggersignal which is delayed through the respective delay line such that twodelayed versions of the trigger signal are created, one to be suppliedfor producing the pre-drive signal and the other for producing the maindrive signal. The optical pulse transmitter 500 may for example berealized with an Er-doped fiber amplifier (EDFA) 510 operating at awavelength of about 1550 nm, or at a wavelength of about 1550±20 nm,where eye-safe operation is possible at a very high peak power. The EDFAis optically pumped with a pump laser 509. The output pulse of the EDFAis indicated in the figure by means of a bold arrow. This optical outputpulse is subsequently used for performing EDM.

Another implementation of the optical pulse transmitter is shown in FIG.5B. Here, only one current source 506 is used which is providing boththe pre-drive and the drive signals to the seed laser. The source 506 iscontrolled by the two pulse generators 504 and 505 like those in theforegoing example.

Further, the optical pulse transmitter 500 typically comprises one ormore microprocessors (not shown) or some other device with computingcapabilities, e.g. an application specific integrated circuit (ASIC), afield programmable gate array (FPGA), a complex programmable logicdevice (CPLD), etc., in order to perform operations such as controllingthe triggering device 501 for initiating drive signals for the seedlaser, or laser diode, 508. When performing steps of differentembodiments of the method of the present invention, the microprocessortypically executes appropriate software that is downloaded to thetransmitter and stored in a suitable storage area, such as e.g. a RAM, aFlash memory or a hard disk. Such a microprocessor or processing unitmay alternatively or optionally be located externally relatively to theoptical pulse transmitter (and electrically connected to the opticalpulse transmitter).

FIG. 6 shows a further example of generated drive signals in accordancewith an embodiment of the present invention. Assuming that the maindrive signal does not fall off with the same high rate with which themain drive signal is rising, a secondary drive signal may be generatedhaving a negative amplitude. The absolute value of the negativeamplitude may for example be comparable to, or higher than, theamplitude of the main drive signal. The secondary drive signal typicallycommences when the pre-drive signal reaches zero or later. Thegeneration of the secondary drive signal facilitates a fast removal ofinjected carriers in the seed laser and therefore shortens pulse width.A secondary drive signal would require a third delay line, pulsegenerator and current source, respectively, in the transmitter of FIG.5A or 5B. The secondary drive signal is indicated in bold in FIG. 6.

FIG. 7 shows an example of an optical pulse transmitter 700 inaccordance with a further embodiment of the present invention. Pulsegenerator 704 produces the pre-drive signal and the main drive signalsupplied to a seed laser, or laser diode, 708 via current source 706.The optical pulse transmitter 700 comprises an EDFA 710 optically pumpedwith a pump laser 709. Since only one pulse generator 704 is used inthis embodiment, the pre-drive signal and the main drive signal will besupplied as a composite signal provided to the seed laser 708 via thesame signal path.

FIG. 8 illustrates an example of a pre-drive signal and a main drivesignal generated in the optical pulse transmitter of FIG. 7.

Even though the invention has been described with reference to specificexemplifying embodiments thereof, many different alterations,modifications and the like will become apparent for those skilled in theart. The described embodiments are therefore not intended to limit thescope of the invention, as defined by the appended claims. Any referencesigns in the claims should not be construed as limiting the scope.

The invention claimed is:
 1. A method of driving a laser diode, themethod comprising: generating a pre-drive signal; generating a maindrive signal, wherein the generating a pre-drive signal generates thepre-drive signal such that the current level of the pre-drive signal iskept at a value equal to or higher than the laser current thresholdvalue for such duration of time that the number of carriers injected inthe laser approaches an equilibrium level before the main drive signalis applied; and applying the pre-drive signal and the main drive signalto the laser diode, wherein said pre-drive signal mitigates spikingeffects in the laser optical output.
 2. The method according to claim 1,wherein the generating a main drive signal generates a duty cycle of themain drive signal such that the duty cycle is less than about 1 percent.3. The method according to claim 1, wherein the generating a pre-drivesignal is generated such that the duration of the pre-drive signal islonger than a characteristic time of the laser diode.
 4. The methodaccording to claim 1, wherein the applying applies the pre-drive signaland the main drive signal such that an amplitude of said pre-drivesignal reaches zero before the amplitude of said main drive signalreaches zero.
 5. The method according to claim 1, wherein the applyingapplies the pre-drive signal and the main drive signal such that anamplitude of said pre-drive signal reaches zero after the amplitude ofsaid main drive signal reaches zero.
 6. The method according to claim 1,further comprising operating the laser diode in one of a single mode andmultiple modes.
 7. An optical pulse transmitter, comprising: a laserdiode; at least one pulse generator configured to generate drive signalsfor driving the laser diode; and at least one current source via whichthe drive pulses are supplied to the laser diode, wherein the pulsegenerator is adapted to generate a pre-drive signal adapted to mitigatespiking effects in the laser optical output, the pre-drive signal beinggenerated such that the current level of the pre-drive signal is kept ata value equal to or higher than the laser current threshold value forsuch duration of time that the number of carriers injected in the laserdiode approaches an equilibrium level before a main drive signal isapplied to the laser diode.
 8. The optical pulse transmitter of claim 7,further comprising an optically pumped amplifier configured to amplify asignal provided by the laser diode and output an optical output signalof the optical pulse transmitter.
 9. The optical pulse transmitter ofclaim 8, wherein the optically pumped amplifier comprises a doped fiberamplifier.
 10. The optical pulse transmitter of claim 9, wherein thedoped fiber amplifier is doped with Nd, Yb, Er, Tm, Pr, Ho or anycombination thereof.
 11. The optical pulse transmitter of claim 9,wherein the doped fiber amplifier is adapted to operate in an eye-safewavelength range.
 12. The optical pulse transmitter according to claim9, wherein the doped fiber-amplifier is adapted to operate within thewavelength interval 1.03-1.10 μm, 1.0-1.1 μm, 1.5-1.6 μm or 1.45-1.53μm, or at a wave-length of about 1.3 μm or 2.1 μm.
 13. The optical pulsetransmitter of claim 7, wherein said pulse generator is adapted togenerate said pre-drive signal such that a resulting optical outputsignal has an energy content that is less than a predetermined fractionof the energy content of an optical output signal resulting from saidmain drive signal.
 14. The optical pulse transmitter of claim 7, whereinsaid pulse generator is adapted to generate said pre-drive signal suchthat a resulting optical output signal has an amplitude not higher thanabout 20% of the amplitude of the output optical signal resulting fromsaid main drive signal.
 15. The optical pulse transmitter of claim 7,wherein said pulse generator is adapted to generate said pre-drivesignal such that a resulting optical output signal has a duration thatis not longer than about 20% of the period of the output optical signalresulting from said main drive signal.
 16. The optical pulse transmitteraccording claim 7, wherein the pulse generator is further adapted togenerate the pre-drive signal such that the duration of the pre-drivesignal is longer than a characteristic time of the laser diode.
 17. Theoptical pulse transmitter of claim 7, further comprising: a triggerdevice configured to generate a trigger signal; and at least twoelectrical delay lines configured to control a timing of the triggersignal generated by the trigger device, wherein, the at least one pulsegenerator includes two pulse generators configured to generate drivesignals for driving the laser diode, one pulse generator generating thepre-drive signal and the other pulse generator generating the main drivesignal, the trigger signal is used for initiating the drive signals tobe generated by the pulse generators, the at least one current sourceincludes two current sources via one of which the pre-drive signal is tobe supplied to the laser diode and via one of which the main drivesignal is to be supplied to the laser diode, and one delay line isconfigured to supply a first delayed trigger signal to the pulsegenerator configured to generate the pre-drive signal, and the otherdelay line is configured to supply a second delayed trigger signal tothe pulse generator configured to generate the main drive signal. 18.The optical pulse transmitter of claim 17, wherein, the at least onepulse generator includes a third pulse generator configured to generatea secondary drive signal for driving the laser diode, the at least onecurrent source includes a third current source via which the secondarydrive signal is to be supplied to the laser diode, and the at least twoelectrical delay lines include a third electrical delay line configuredto supply a third delayed trigger signal to the third pulse generatorconfigured to generate the secondary drive signal, thereby facilitatingfast decrease of current through the laser diode.
 19. The optical pulsetransmitter of claim 18, wherein the secondary drive signal has anegative amplitude, the absolute value of the negative amplitude beingequal to or greater than the amplitude of the main drive signal, and thethird pulse generator is configured to supply the secondary drive signalwhen the pre-drive signal reaches zero or later.
 20. A total stationcomprising the optical pulse transmitter according to claim
 7. 21. Anon-transitory computer-readable digital storage medium comprising acomputer program product comprising computer-executable componentsadapted to, when executed on a processing unit of a device, cause thedevice to perform the method according to claim 1.